Rolled material for high strength spring, and wire for high strength spring

ABSTRACT

It is an object of the present invention to provide a rolled material, which is a material for high strength spring, and also can exhibit excellent corrosion fatigue properties after quenching and tempering even when suppressing the addition amount of an alloying element; and a wire for high strength spring obtained from such a rolled material. The rolled material for high strength spring of the present invention includes, in % by mass: C: 0.39 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%, P: exceeding 0% and 0.015% or less, S: exceeding 0% and 0.015% or less, Al: 0.001 to 0.1%, Cu: 0.10 to 0.80%, Ni: 0.10 to 0.80%, and O: exceeding 0% and 0.0010% or less, with the balance being iron and inevitable impurities, wherein the number of oxide inclusions having an average diameter of 25 μm or more is 30 or less per 100 g of a steel material, and an amount of nondiffusible hydrogen is 0.40 ppm by mass or less.

FIELD OF THE INVENTION

The present invention relates to a rolled material for high strengthspring, and a wire for high strength spring using the same. Moreparticularly, the present invention relates to a rolled material and awire for high strength spring, which are useful as raw materials of highstrength springs that are used in a state of being subjected torefining, namely, quenching and tempering, particularly a rolledmaterial having excellent in corrosion fatigue properties after quenchand temper, which are excellent in corrosion fatigue properties eventhough a tensile strength is a high strength in a range of 1,900 MPa ormore after wire drawing.

BACKGROUND ART

Coil springs used in automobiles, for example, a valve spring and asuspension spring used in the engine, suspension, and the like arerequired to reduce the weight and to increase the strength so as toachieve exhaust gas reduction and improvement in fuel economy. Thespring imparted with high strength is likely to cause hydrogenbrittleness because of its poor toughness and ductility, leading todegradation of corrosion fatigue properties. Therefore, the steel wire(hereinafter, the steel wire is sometimes referred to as a wire) forhigh strength spring used in the manufacture of a spring is required tohave excellent corrosion fatigue properties. Hydrogen generated bycorrosion enters into a steel and may lead to embrittlement of a steelmaterial, thus causing corrosion fatigue fracture, so that there is aneed to improve corrosion resistance and hydrogen embrittlementresistance of the steel material so as to improve corrosion fatigueproperties.

There has been known, as a method for enhancing corrosion fatigueproperties of a wire for high strength spring, a method for controllingthe chemical composition. However, such a method is not necessarilydesirable from a viewpoint of an increase in manufacturing costs andresource saving because of use of a large amount of an alloying element.

Meanwhile, there have been known, as a method for manufacturing aspring, a method in which a steel wire is heating to a quenchingtemperature and hot-formed into a spring shape, followed by oil coolingand further tempering, and a method in which a steel wire is subjectedto quenching and tempering, and then cold-formed into a spring shape. Inthe cold forming method of the latter, it is also known that quenchingand tempering before forming is performed by high frequency inductionheating. For example, Patent Document 1 discloses technology in which awire material is cold-drawn and then the structure is adjusted byquenching and tempering through high frequency induction heating.According to this technology, a structural fraction of pearlite is setat 30% or less and a structural fraction composed of martensite andbainite is set at 70% or more and then cold drawing is performed at apredetermined area reduction rate, followed by quenching and temperingto thereby reduce the insoluble carbides, leading to an improvement indelayed fracture properties.

In Patent Document 2, a rolled wire material is subjected to wiredrawing, followed by a quenching and tempering treatment through highfrequency induction heating in Examples. This technology focusesprimarily on achievement of the reconciliation of high strength andcoiling properties, and gives no consideration to corrosion fatigueproperties.

While paying attention to the amount of hydrogen in a steel that isevaluated by the total amount of hydrogen released when the temperatureis raised from room temperature to 350° C., Patent Document 3 proposes ahot rolled wire material having excellent wire drawability under highdegree wire drawing conditions. However, Patent Document 3 focuses onlyon wire drawability during special processing such as high degree wiredrawing, and also gives no consideration to corrosion fatigue propertiesafter quenching and tempering, which becomes most important in asuspension spring.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP 2004-143482 A-   Patent Document 2: JP 2006-183137 A-   Patent Document 3: JP 2007-231347 A

SUMMARY OF INVENTION Problems to be Solved by the Invention

In light of aforementioned circumstances, the present invention has beenmade, and it is an object thereof is to provide a rolled material, whichis a material for high strength spring of hot coiling and cold coiling,and also can exhibit excellent corrosion fatigue properties afterquenching and tempering even when suppressing the addition amount of analloying element; and a wire for high strength spring obtained from sucha rolled material.

Means for Solving the Problems

The present invention that can solve the foregoing problems provides arolled material for high strength spring, including, in % by mass:

C: 0.39 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%,

P: exceeding 0% and 0.015% or less,S: exceeding 0% and 0.015% or less,

Al: 0.001 to 0.1%, Cu: 0.10 to 0.80%, Ni: 0.10 to 0.80%, and

O: exceeding 0% and 0.0010% or less, with the balance being iron andinevitable impurities, wherein

the number of oxide inclusions having an average diameter of 25 μm ormore is 30 or less per 100 g of a steel material, and an amount ofnondiffusible hydrogen is 0.40 ppm by mass or less.

When an average diameter of oxide inclusions is determined, a major axisand a minor axis of an oxide inclusion are respectively measured byobserving using an electron probe micro analyzer (EPMA), and an averageof the major axis and the minor axis of the oxide inclusion, namely, avalue obtained by dividing the sum of the major axis and the minor axisby 2 is regarded as an average diameter. Inclusions exhibiting thisaverage of 25 μm or more are objects that are subjected to measurementof the number in the present invention.

It is preferable that the rolled material for high strength spring ofthe present invention further includes, in % by mass, at least one offollowing (a) to (d):

(a) Cr: exceeding 0% and 1.2% or less,(b) Ti: exceeding 0% and 0.13% or less,(c) B: exceeding 0% and 0.01% or less, and(d) at least one of Nb: exceeding 0% and 0.1% or less, andMo: exceeding 0% and 0.5% or less.

The present invention also include a wire for high strength spring,including any of chemical components of the steel mentioned above,wherein an area ratio of tempered martensite is 80% or more, and atensile strength is 1,900 MPa or more.

Effects of the Invention

According to the present invention, oxide inclusions in the rolledmaterial are reduced and the amount of nondiffusible hydrogen issuppressed without adding a large amount of an alloying element, thusmaking it possible to exhibit excellent corrosion fatigue propertiesafter quenching and tempering. In such a rolled material, it is possibleto improve corrosion fatigue properties of the wire even whensuppressing the cost of steel materials, thus making it possible tosupply a high strength spring which is very unlikely to cause corrosionfatigue fracture, for example, a coil spring such as a suspension springthat is one of automobile components, at a cheap price.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an influence of the number of inclusions andan amount of nondiffusible hydrogen in a rolled material on corrosionfatigue properties.

MODE FOR CARRYING OUT THE INVENTION

With the progress of corrosion of a wire, pits are generated on asurface of a wire rod, and also a wire diameter of the wire roddecreases due to thinning caused by corrosion. Hydrogen generated bycorrosion enters into a steel, leading to embrittlement of the steelmaterial. Corrosion fatigue fracture occurs from these corrosion pits,thinning positions and embrittled regions of steel materials as startingpoints. Therefore, corrosion fatigue fracture can be suppressed byimproving hydrogen embrittlement resistance and corrosion resistance ofthe wire rod.

Inventors of the present invention have made a study of factors, whichaffect hydrogen embrittlement resistance and corrosion resistance fromevery angle. As a result, it became apparent that corrosion fatigueproperties are significantly improved by subjecting a rolled material,in which both the number of oxide inclusions having a predetermined sizein the steel and the amount of hydrogen in the steel, especially theamount of nondiffusible hydrogen, to quenching and tempering treatment.Consequently, the inventors have found that, if numerous large oxideinclusions exist in the steel, not only durability in the atmosphere isdegraded, but also “strain field” is formed around the oxide inclusions,leading to formation of the hydrogen accumulation place, thus causingembrittlement of grain boundaries around them and degradation ofcorrosion fatigue properties.

Appropriate control of the amount of oxide inclusions and the amount ofhydrogen enables an improvement in corrosion fatigue properties even ifthe addition amount of a corrosion resistance-improving element isreduced. Hereinafter, description will be made of requirements definedin the present invention, for example, the number of oxide inclusions,the amount of nondiffusible hydrogen in the steel, and chemicalcomposition.

Number of Oxide Inclusions

If numerous large oxide inclusions exist in the steel, not onlydurability in the atmosphere is degraded, but also the strain field isformed around the oxide inclusions, leading to formation of the hydrogenaccumulation place, thus causing embrittlement of grain boundariesaround them and degradation of corrosion fatigue properties. To reducean adverse influence on corrosion fatigue properties, there is a needthat the number of oxide inclusions having an average diameter of 25 μmor more is set at 30 or less per 100 g of a steel material (hereinaftersometimes referred to as “30 or less per 100 g). The number of oxideinclusions is preferably 20 or less per 100 g, and more preferably 10 orless per 100 g. To improve corrosion fatigue properties, there is noneed to set the lower limit of the number of oxide inclusions. However,there will be production costs for setting the lower limit at 0 per 100g, so that the lower limit is preferably 2 or more per 100 g in view ofindustrial production. Oxide inclusions having an average diameter of 25μm or more serve as fracture starting point that is a stressconcentration source, thus degrading corrosion fatigue properties,whereas, oxide inclusions having an average diameter of less than 25 μmdo not exert an adverse influence on corrosion fatigue properties.

Amount of Nondiffusible Hydrogen

In the rolled material of the present invention, there is a need thatthe amount of nondiffusible hydrogen is set at 0.40 ppm by mass or less.If a large amount of nondiffusible hydrogen exists in the rolledmaterial, the amount of nondiffusible hydrogen also increases in thewire after quenching and tempering. If a large amount of nondiffusiblehydrogen exists in the wire, a permissible amount of hydrogen, whichfurther enters until the steel material embrittles, decreases.Therefore, even though a small amount of hydrogen entered during use asa spring, embrittlement of the steel material occurs and early fractureis likely to occur, resulting in degraded hydrogen embrittlementresistance. The amount of nondiffusible hydrogen is preferably 0.35 ppmby mass or less, and more preferably 0.30 ppm by mass or less. The lessthe amount of nondiffusible hydrogen, the better. However, it isdifficult to set at 0 ppm by mass and the lower limit is about 0.01 ppmby mass.

The amount of nondiffusible hydrogen is an amount of hydrogen measuredby the method mentioned in Examples below, and specifically means thetotal amount of hydrogen released at 300 to 600° C. when the temperatureof a steel material is raised at 100° C./hour.

The rolled material for high strength spring according to the presentinvention is a low alloy steel in which the content of an alloyingelement is suppressed, and the chemical composition is as follows. Thepresent invention also includes a wire obtained by wire-drawing theabove-mentioned rolled material, followed by quenching and tempering,and the chemical composition is the same as that of the rolled material.As used herein, chemical composition means % by mass.

C: 0.39 to 0.65%

C is an element that is required to ensure the strength of a wire forspring, and is also required to generate fine carbides that serve ashydrogen trapping sites. From such a viewpoint, the amount of C wasdetermined in a range of 0.39% or more. The lower limit of the amount ofC is preferably 0.45% or more, and more preferably 0.50% or more.Excessive C amount, however, might generate coarse residual austeniteand non-solid soluted carbides after quenching and tempering, whichfurther degrades hydrogen embrittlement resistance. C is an element thatdegrades corrosion resistance, so that there is a need to suppress theamount of C so as to enhance corrosion fatigue properties of a springproduct such as a suspension spring which is a final product. From sucha viewpoint, the amount of C was determined in a range of 0.65% or less.The upper limit of the amount of C is preferably 0.62% or less, and morepreferably 0.60% or less.

Si: 1.5 to 2.5%

Si is an element that is required to ensure the strength of a wire forspring, and also exhibits the effect of refining carbides. Toeffectively exhibit these effects, the amount of Si was determined in arange of 1.5% or more. The lower limit of the amount of Si is preferably1.7% or more, and more preferably 1.9% or more. Meanwhile, since Si isalso an element that accelerates decarburization, excessive Si amountaccelerates formation of a decarburized layer on a surface of a wirerod, thus requiring the peeling step for removal of the decarburizedlayer, resulting in increased manufacturing costs. Non-solid solutioncarbides also increase, thus degrading hydrogen embrittlementresistance. From such a viewpoint, the amount of Si was determined in arange of 2.5% or less. The upper limit of the amount of Si is preferably2.3% or less, more preferably 2.2% or less, and still more preferably2.1% or less.

Mn: 0.15 to 1.2%

Mn is an element that is employed as a deoxidizing element and reactswith S, which is a harmful element in a steel, to form MnS, and isuseful for detoxication of S. Mn is also an element that contributes toan improvement in strength. To effectively exhibit these effects, theamount of Mn was determined in a range of 0.15% or more. The lower limitof the amount of Mn is preferably 0.2% or more, and more preferably 0.3%or more. Excessive Mn amount, however, degrades toughness, thus causingembrittlement of a steel material. From such a viewpoint, the amount ofMn was determined in a range of 1.2% or less. The upper limit of theamount of Mn is preferably 1.0% or less, and more preferably 0.85% orless.

P: Exceeding 0% and 0.015% or Less

P is a harmful element that degrades ductility such as coilingproperties of a rolled material such as a wire rod, and the amountthereof is preferably as small as possible. is likely to segregate ingrain boundaries to cause grain boundary embrittlement, and hydrogen islikely to cause fracture of grain boundaries, thus exerting an adverseinfluence on hydrogen embrittlement resistance. From such a viewpoint,the amount of P was determined in a range of 0.015% or less. The upperlimit of the amount of P is preferably 0.010% or less, and morepreferably 0.008% or less. The amount of P is preferably as small aspossible, and is contained usually about 0.001%.

S: Exceeding 0% and 0.015% or Less

Like P mentioned above, S is a harmful element that degrades ductilitysuch as coiling properties of a rolled material, and the amount thereofis preferably as small as possible. S is likely to segregate in grainboundaries to cause grain boundary embrittlement, and hydrogen is likelyto cause fracture of grain boundaries, thus exerting an adverseinfluence on hydrogen embrittlement resistance. From such a viewpoint,the amount of S was determined in a range of 0.015% or less. The upperlimit of the amount of S is preferably 0.010% or less, and morepreferably 0.008% or less. The amount of S is preferably as small aspossible, and is usually contained about 0.001%.

Al: 0.001 to 0.1%

Al is mainly added as a deoxidizing element. This element reacts with Nto form AlN to thereby detoxicate solid-soluted N, and also contributesto refining of the structure. To adequately exhibit these effects, theamount of Al was determined in a range of 0.001% or more. The lowerlimit of the amount of Al is preferably 0.002% or more, and morepreferably 0.005% or more. However, since Al is an element thataccelerates decarburization, like Si, there is a need to suppress theamount of Al in a steel for spring, which includes a large amount of Si.Therefore, in the present invention, the amount of Al was determined ina range of 0.1% or less. The upper limit of the amount of Al ispreferably 0.07% or less, more preferably 0.030% or less, andparticularly preferably 0.020% or less.

Cu: 0.10 to 0.80%

Cu is an element that is effective in suppressing surfacedecarburization and improving corrosion resistance. Therefore, theamount of Cu was determined in a range of 0.10% or more. The lower limitof the amount of Cu is preferably 0.15% or more, and more preferably0.20% or more. Excessive Cu amount, however, causes cracks during hotworking and increases costs. Therefore, the amount of Cu was determinedin a range of 0.80% or less. The upper limit of the amount of Cu ispreferably 0.70% or less, and more preferably 0.60% or less. The amountof Cu is preferably 0.48% or less, 0.35% or less, and 0.30% or less.

Ni: 0.10 to 0.80%

Like Cu, Ni is an element that is effective in suppressing surfacedecarburization and improving corrosion resistance. Therefore, theamount of Ni was determined in a range of 0.10% or more. The lower limitof the amount of Ni is preferably 0.15% or more, and more preferably0.20% or more. Excessive Ni amount, however, increases costs. Therefore,the amount of Ni was determined in a range of 0.80% or less. The upperlimit of the amount of Ni is preferably 0.70% or less, and morepreferably 0.60% or less. The amount of Ni is preferably 0.48% or less,0.35% or less, and 0.30% or less.

O: Exceeding 0% and 0.0010% or Less

If oxygen exists in a steel material, oxide inclusions such as Al₂O₃,SiO₂, CaO, MgO and TiO₂ are formed. Oxide inclusions are hard, and thusstrain is generated around oxide inclusions due to a difference inhardness with a material around oxide inclusions. Hydrogen accumulatedon the strain causes embrittlement of grain boundaries around the oxideinclusions. Therefore, reducing the amount of oxygen is important toimprove corrosion fatigue properties. Therefore, the upper limit of theamount of O was set at 0.0010% or less. The upper limit is preferably0.0008% or less, and more preferably 0.0006% or less. Whereas, the lowerlimit of the amount of O is generally 0.0002% or more on industrialproduction.

Basic components of the rolled material of the present invention are asmentioned above, the balance being substantially iron. As a matter ofcourse, inclusion of inevitable impurities such as Ca, Mg and Nintroduced by the state of raw material, material, manufacturingfacility, and the like is permitted. The rolled material for spring ofthe present invention has the chemical composition mentioned above andcan achieve excellent coiling properties and hydrogen embrittlementresistance while having high strength. Elements mentioned below may befurther included for the purpose of improving corrosion resistanceaccording to application.

Cr: Exceeding 0% and 1.2% or Less

Cr is an element that is effective in improving corrosion resistance. Toeffectively exhibit these effects, the amount of Cr is preferably 0.05%or more, more preferably 0.08% or more, and still more preferably 0.10%or more. However, Cr is an element that has a strong tendency to formcarbides, and forms peculiar carbides in a steel material and is likelyto be dissolved in cementite in a high concentration. It is effective toinclude a small amount of Cr, however, the heating time of the quenchingstep decreases in high frequency induction heating, leading toinsufficient austenitizing of dissolving carbide, cementite, and thelike into a matrix. Therefore, when including a large amount of Cr,dissolving residue of cementite, in which Cr-based carbide and metallicCr are solid-saluted in high concentration is generated as a stressconcentration source, so that fracture likely to occur, thus degradinghydrogen embrittlement resistance. Therefore, the amount of Cr ispreferably 1.2% or less, more preferably 0.8% or less, and still morepreferably 0.6% or less.

Ti: Exceeding 0% and 0.13% or Less

Ti is an element that is useful to react with S to form sulfide tothereby detoxicate S. Ti also has the effect of refining the structureby forming carbonitride. To effectively exhibit these effects, theamount of Ti is preferably 0.02% or more, more preferably 0.05% or more,and still more preferably 0.06% or more. Excessive Ti amount, however,may form coarse Ti sulfide, thus degrading ductility. Therefore, theamount of Ti is preferably 0.13% or less. From a viewpoint of costreduction, the amount of Ti is preferably 0.10% or less, and morepreferably 0.09% or less.

B: Exceeding 0% and 0.01% or Less

B is an element that improve hardenability and strengthens prioraustenite crystal grain boundaries, and also contributes to suppressionof fracture. To effectively exhibit these effects, the amount of B ispreferably 0.0005% or more, and more preferably 0.0010% or more.Excessive B amount, however, causes saturation of the above effects, sothat the amount of B is preferably 0.01% or less, more preferably0.0050% or less, and still more preferably 0.0040% or less.

At Least One of Nb: Exceeding 0% and 0.1% or Less, and Mo: Exceeding 0%and 0.5% or Less

Nb is an element that forms carbonitride together with C and N, andmainly contributes to refining of the structure. To effectively exhibitthese effects, the amount of Nb is preferably 0.003% or more, morepreferably 0.005% or more, and still more preferably 0.01% or more.Excessive Nb amount, however, form coarse carbonitride, thus degradingductility of a steel material. Therefore, the amount of Nb is preferably0.1% or less. From a viewpoint of cost reduction, the amount ispreferably set at 0.07% or less.

Like Nb, Mo is also an element that forms carbonitride together with Cand N, and contributes to refining of the structure. Mo is an elementthat is also effective in ensuring the strength after tempering. Toeffectively exhibit these effects, the amount of Mo is preferably 0.15%or more, more preferably 0.20% or more, and still more preferably 0.25%or more. Excessive Mo amount, however, form coarse carbonitride, thusdegrading ductility, for example, coiling properties of a steelmaterial. Therefore, the amount of Mo is preferably 0.5% or less, andmore preferably 0.4% or less.

Nb and Mo may be included individually, or both of them may be includedin combination. The rolled material of the present invention includes Nas inevitable impurities, and the amount of it is preferably adjusted ina range mentioned below.

N: Exceeding 0% and 0.007% or Less

N is an element included in inevitable impurities. As the amount of Nincreases, it forms coarse nitride together with Ti and Al, thusexerting an adverse influence on fatigue properties. Therefore, theamount of N is preferably as small as possible. The amount of N forexample, 0.007% or less, and more preferably 0.005% or less. Meanwhile,if the amount of N is too reduced, productivity is drastically degraded.N forms nitride together with Al to thereby contribute to refining ofcrystal grains. From such a viewpoint, the amount of N is preferably0.001% or more, more preferably 0.002% or more, and still morepreferably 0.003% or more.

A method for producing a rolled material of the present invention willbe described below. In a series of steps of melting a steel having theabove chemical composition, followed by continuous casting, blooming andhot rolling, it is possible to control the amount of nondiffusiblehydrogen of the rolled material by adjusting at least one of (A) theamount of hydrogen in a molten steel stage, (B) the homogenizingtreatment temperature and time before blooming, and (C) the cooling ratein a range of 400 to 100° C. after hot rolling.

There is a need to remove hydrogen in a steel by diffusion so as toreduce hydrogen in the steel after solidification, and heating at a hightemperature for a long time is effective to increase a diffusion rate ofhydrogen so as to release hydrogen from a surface of a steel material.Specific examples of the method of reducing the amount of hydrogen inthe steel include a method of adjusting in a molten steel stage, amethod of adjusting in a stage of a continuously cast material at 1,000°C. or higher after solidification, a method of adjusting in a heatingstage before hot rolling, a method of adjusting in a heated stage duringhot rolling, and a method of adjusting in a cooling stage after rolling.It is particularly preferred to perform at least one of treatments forreducing nondiffusible hydrogen (A) to (C) mentioned below.

(A) A degassing treatment is performed in a melting steel process tothereby adjust the amount of hydrogen in a molten steel at 2.5 ppm bymass or less.

For example, it is effective that a vacuum tank equipped with twoimmersion tubes mounted in a ladle in a secondary refining step and thenan Ar gas is blown from the side of one immersion tube, followed byvacuum degassing that enables circulation of a molten steel to thevacuum tank utilizing the buoyancy. This method is excellent in hydrogenremoving capability and reduction in inclusion. The amount of hydrogenin the molten steel is preferably 2.0 ppm by mass or less, morepreferably 1.5 ppm by mass or less, and particularly preferably 1.0 ppmby mass or less.

(B) A homogenizing treatment (heating) before blooming is performed at1,100° C. or higher, and preferably 1,200° C. or higher for 10 hours ormore.(C) An average cooling rate in a range of 400 to 100° C. after hotrolling is set at 0.5° C./second or less, and preferably 0.3° C./secondor less.

When a steel material has a large cross-sectional area, particularly, itbecomes necessary to perform heating for a long time. If the steelmaterial is heated for a long time, decarburization is accelerated, sothat the amount of hydrogen in the steel is preferably reduced byperforming the treatment (A) mentioned above.

There is no particular limitation on the coiling temperature TL afterhot rolling, and cooling conditions beyond a temperature range of 400 to100° C. after coiling.

The coiling temperature TL can be set, for example, in a range of 900°C. or higher and 1,000° C. or lower, and is preferably 910° C. orhigher, and more preferably 930° C. or higher. An average cooling rateat the coiling temperature of TL to 650° C. can be set in a range of 2°C./second or more and 5° C./second or less. The lower limit of theaverage cooling rate at the coiling temperature of TL to 650° C. ispreferably 2.3° C./second or more, and more preferably 2.5° C./second ormore. The upper limit of the average cooling rate at the coilingtemperature of TL to 650° C. is preferably 4.5° C./second or less, andmore preferably 4° C./second or less. Further, the average cooling rateat 650 to 400° C. can be set at 2° C./second or less. The averagecooling rate at 650 to 400° C. is preferably 1.5° C./second or less, andmore preferably 1° C./second or less. There is no particular limitationon the lower limit of the average cooling rate, and the lower limit is,for example, about 0.3° C./second.

Reduction in Oxide Inclusions

To reduce oxide inclusions, there is a need to set the content of oxygenof the wire rod at a defined value or less. Sufficient oxidizing withaluminum and silicon as well as sufficient degassing enable reduction ininclusions, leading to achievement of higher cleanliness and reductionin oxide inclusions.

To manufacture a coil spring used in automobiles, there is a need that awire is manufactured by wire processing, namely, wire drawing of therolled material mentioned above. For example, in a cold coiled spring,quenching and tempering by high frequency induction heating areperformed after wire drawing, and such a wire is also included in thepresent invention.

A high strength wire having a tensile strength of 1,900 MPa or more canbe obtained by subjecting the rolled material to wire working, namely,wire drawing, followed by quenching and tempering by high frequencyinduction heating. Specifically, the rolled material is subjected towire drawing at an area reduction rate of about 5 to 35%, followed byquenching at about 900 to 1,000° C. and further tempering at about 300to 520° C. The quenching temperature is preferably 900° C. or higher soas to sufficiently perform austenitizing, and preferably 1,000° C. orlower so as to prevent grain coarsening. The heating temperature fortempering may be set at an appropriate temperature in a range of 300 to520° C. according to a target value of a wire strength. When quenchingand tempering are performed by high frequency induction heating,quenching and tempering times are respectively in a range of about 10 to60 seconds.

Regarding the structure after quenching and tempering, there is a needthat the tempered martensite structure has 80 area or more. As a resultof increasing the proportions of non-solid-soluted ferrite and residualaustenite, the strength decreases. Regarding the structure afterquenching and tempering, the tempered martensite structure preferablyhas 88 area % or more. To set the proportion of the tempered martensitestructure at 80 area % or more, it is preferable that the material isheated to 900° C. or higher when heating before quenching, followed bysufficient austenitizing and further cooling to 100° C. or lower bywater cooling or oil cooling.

The thus obtained wire of the present invention can realize a hightensile strength in a range of 1,900 MPa or more. The tensile strengthis usually selected in a range of 1,900 MPa to 2,200 MPa. Although thereis no particular limitation on the upper limit of the tensile strength,and the upper limit is about 2,500 MPa. The wire of the presentinvention can exhibit corrosion fatigue properties even at a highstrength in a range of 1,900 MPa or more because of use of the rolledmaterial of the present invention.

This application claims priority based on Japanese Patent ApplicationNo. 2014-039368 filed on Feb. 28, 2014, the disclosure of which isincorporated by reference herein.

The present invention will be described in more detail below by way ofExamples. It should be noted that, however, these examples are neverconstrued to limit the scope of the invention; various modifications andchanges may be made without departing from the scope and spirit of theinvention and should be considered to be within the scope of theinvention.

Examples

Each of steel materials having chemical compositions shown in Tables 1to 3 was melted by melting in a converter and then subjecting tocontinuous casting and a homogenizing treatment at 1,100° C. or higher.After the homogenizing treatment, blooming was performed, followed byheating at 1,000 to 1,280° C. and further hot rolling to obtain a rolledmaterial having a diameter of 14.3 mm, namely, a wire rod. It is asshown in Tables 4 to 6 below whether or not a degassing treatment of amolten steel by the above-mentioned material is implemented, and whetheror not cooling is implemented after coiling, namely, whether or notcooling at an average cooling rate of 0.5° C./second or less isimplemented at 400 to 100° C. after rolling. The amount of 0 in themolten steel shown in Tables 4 to 6 was adjusted by controlling thedegree of deoxidizing with aluminum and silicon.

The coiling temperature TL after hot rolling was set at 950° C., andother cooling after coiling was performed at an average cooling rate of4° C./second at a temperature in a range of TL to 650° C., and performedat an average cooling rate of 1° C./second at a temperature in a rangeof 650 to 400° C. In test examples in which “Implementation” is writtenin the column of the homogenizing treatment, the homogenizing treatmentis performed at 1,100° C. for 10 hours or more. In test examples inwhich the mark “-” is written, the time of the homogenizing treatment at1,100° C. is less than 10 hours.

Regarding the wire rod thus obtained, the amount of nondiffusiblehydrogen and the number of oxide inclusions were measured by thefollowing procedures. The results are shown in Table 4 to 6. In Tables 4to 6, the number of oxide inclusions having an average diameter of 25 μmor more in the rolled material was written as “Number of inclusions of25 μm or more of rolled material”.

Amount of Nondiffusible Hydrogen

A specimen measuring 20 mm in width×40 mm in length was cut out from therolled material, namely, wire rod. After raising the temperature of thespecimen at a temperature rise rate of 100° C./hour, a hydrogen releaseamount at 300 to 600° C. was measured using a gas chromatogram, and thehydrogen release amount was regarded as the amount of nondiffusiblehydrogen.

Number of Oxide Inclusions

The number of oxide inclusions was determined by the followingprocedure: an average of the results of examination of six rolledmaterial samples each having 50 g in weight was determined, followed byconversion into the number per 100 g. The number of inclusions wasexamined by an acid dissolution method. Each sample (50 g) was dissolvedwith an acid and inclusions, remaining without being dissolved, wasallowed to remain on a filter paper. Inclusions having an averagediameter of 25 μm or more were sorted by EPMA, analyzed by energydispersive X-ray spectrometry (EDX), and then oxide inclusions weresorted. Regarding the above-mentioned six samples, the number of oxideinclusions having an average diameter of 25 μm or more was measured andan average thereof was determined, followed by conversion into thenumber per 100 g of a steel material. In this case, nitric acid adjustedso as not to dissolve oxide inclusions was used for dissolution with anacid. An average diameter of oxide inclusions means an average of amajor axis and a minor axis, namely, the value obtained by dividing thesum of the major axis and the minor axis by 2. To reduce the number ofoxide inclusions, oxygen was removed by sufficiently perform vacuumdegassing when melting in a converter.

Then, the wire rod was subjected to wire drawing, namely, cold drawingto thereby reduce to a diameter of 12.5 mm, followed by quenching andtempering. An area reduction rate of wire drawing is about 23.6%, andthe conditions of quenching and tempering are as follows.

Quenching and Tempering Conditions

High frequency induction heating

Heating rate: 200° C./second

Quenching: 950° C., 20 seconds, water cooling

Tempering: each temperature in a range of 300 to 520° C., 20 seconds,water cooling

It is possible to obtain a structure in which an area ratio of temperedmartensite is 80% or more, by quenching and tempering mentioned above.In this test, it was confirmed that the area ratio of the entiretempered martensite is 80% or more.

Regarding the wire after wire drawing, and quenching and tempering, thetensile strength and corrosion fatigue properties were evaluated. Theresults are collectively shown in Tables 4 to 6 below.

Measurement of Tensile Strength

After quenching and tempering, a wire was cut into a predeterminedlength and a tensile test was performed at a distance between chucks of200 mm and a tensile speed of 5 mm/minute in accordance with JIS Z2241(2011).

Evaluation of Corrosion Fatigue Properties

Corrosion fatigue properties were evaluated by fracture life aftersubjecting to a corrosion treatment and performing the Ono-typerotating-bending fatigue test. Each wire subjected to quenching andtempering was cut to fabricate a No. 1 test specimen (JIS Z 2274(1978)). The parallel part of this test specimen was polished using asand paper of No. 800. A test was carried out without shot peening of asurface. First, the test specimen thus processed was subjected to acorrosion treatment under the following conditions.

Corrosion Treatment

Using an aqueous 5% NaCl solution at 35° C., salt spraying was performedfor 8 hours, followed by drying and holding in a wet atmosphere at 35°C. and relative humidity of 60% for 16 hours (1 cycle). The testspecimen was subjected to a corrosion treatment by repeating 10 cyclesin total. After the corrosion treatment, the test specimen was subjectedto the rotating-bending test and then corrosion fatigue properties wereevaluated. Using ten test specimens for each test, load stress was setat 500 MPa and the Ono-type rotating-bending fatigue test was carriedout. Measurement was made of fatigue life until fracture of each testspecimen occurs. An average of each fatigue life of ten test specimenswas measured. The case where the average of fatigue life is 100,000times or more was rated as excellent corrosion fatigue life.

TABLE 1 Steel Chemical composition (% by mass) The balance being ironand inevitable impurities No. C Si Mn P S Al Cu Ni Cr Ti B Nb Mo N 10.42 2.1 1.05 0.008 0.006 0.021 0.22 0.23 0.35 0.090 — — — 0.0039 2 0.431.8 0.77 0.005 0.007 0.020 0.21 0.24 0.30 0.120 — — — 0.0049 3 0.40 2.00.65 0.007 0.012 0.035 0.35 0.40 0.27 0.080 — — — 0.0040 4 0.44 2.1 0.950.012 0.007 0.003 0.30 0.22 0.36 0.070 — — — 0.0055 5 0.60 1.7 0.800.004 0.008 0.022 0.35 0.30 — — 0.0030 — — 0.0043 6 0.59 2.0 0.90 0.0060.007 0.021 0.35 0.32 — — 0.0035 — — 0.0034 7 0.62 2.0 0.65 0.010 0.0110.020 0.29 0.30 0.15 — 0.0020 — — 0.0054 8 0.57 2.1 0.66 0.008 0.0070.022 0.33 0.36 0.23 — 0.0019 — — 0.0047 9 0.60 2.0 0.80 0.005 0.0060.033 0.35 0.30 0.08 0.090 — — — 0.0054 10 0.59 1.6 0.50 0.008 0.0060.002 0.27 0.27 0.32 0.070 — — — 0.0043 11 0.55 2.0 0.94 0.004 0.0040.040 0.27 0.27 0.32 0.040 0.0030 — — 0.0030 12 0.48 2.0 0.45 0.0070.007 0.002 0.32 0.32 0.35 0.080 — — — 0.0052 13 0.62 1.5 0.95 0.0060.008 0.045 0.26 0.26 0.75 — — — — 0.0037 14 0.63 2.2 0.53 0.006 0.0060.021 0.21 0.45 0.26 — — 0.08 — 0.0043 15 0.58 2.0 0.20 0.010 0.0110.020 0.14 0.14 0.19 — — — 0.25 0.0038 16 0.54 2.1 0.70 0.007 0.0070.022 0.45 0.33 0.38 — — — — 0.0025

TABLE 2 Steel Chemical composition (% by mass) The balance being ironand inevitable impurities No. C Si Mn P S Al Cu Ni Cr Ti B Nb Mo N 170.43 2.0 0.65 0.004 0.002 0.021 — — 0.25 — — — — 0.0057 18 0.50 1.7 0.670.004 0.002 0.022 0.08 0.07 0.35 — — — — 0.0045 19 0.47 2.0 0.95 0.0050.008 0.020 0.21 0.25 0.36 0.050 — — — 0.0031 20 0.48 2.0 0.89 0.0040.002 0.020 0.45 0.30 0.36 — — — — 0.0043 21 0.58 2.1 0.78 0.004 0.0020.021 0.42 0.40 — — 0.0044 — — 0.0032 22 0.61 1.9 0.52 0.008 0.005 0.0200.41 0.29 — — 0.0049 — — 0.0043 23 0.50 2.1 0.59 0.008 0.008 0.023 0.430.43 0.48 0.020 — 0.02 — 0.0041 24 0.58 1.9 0.78 0.004 0.004 0.020 0.180.18 0.23 0.080 — — — 0.0043 25 0.42 1.6 0.83 0.008 0.008 0.023 0.430.43 0.48 — — — — 0.0036 26 0.48 2.0 0.50 0.008 0.009 0.022 0.33 0.330.38 — — — — 0.0047 27 0.52 2.1 0.51 0.005 0.006 0.021 0.24 0.24 0.29 —— — — 0.0049 28 0.55 2.0 0.86 0.008 0.008 0.022 0.35 0.35 0.40 — — — —0.0043 29 0.60 1.5 0.63 0.010 0.011 0.022 0.38 0.38 0.43 — — — — 0.002830 0.43 1.9 0.90 0.005 0.005 0.020 0.19 0.19 0.24 — — — — 0.0052 31 0.501.9 0.93 0.009 0.005 0.020 0.19 0.19 0.24 — — — — 0.0048

TABLE 3 Steel Chemical composition (% by mass) The balance being ironand inevitable impurities No. C Si Mn P S Al Cu Ni Cr Ti B Nb Mo N 320.57 2.2 0.34 0.006 0.008 0.028 0.29 0.48 0.30 0.10 — — — 0.0039 33 0.522.1 0.53 0.007 0.008 0.028 0.30 0.50 0.28 0.09 0.0030 — — 0.0041 34 0.602.2 0.40 0.006 0.008 0.025 0.31 0.58 0.34 0.08 0.0030 — — 0.0052 35 0.542.1 0.57 0.010 0.010 0.033 0.28 0.79 0.26 0.07 — — — 0.0039 36 0.57 2.10.40 0.008 0.009 0.028 0.12 0.54 0.27 0.11 — — — 0.0042 37 0.58 2.2 0.710.007 0.006 0.031 0.21 0.58 0.27 0.08 — — — 0.0041 38 0.57 2,2 0,610.008 0.010 0.027 0.57 0.61 0.27 0.09 — — — 0.0053 39 0.60 2.3 0,470.008 0.008 0.024 0.28 0.54 0.31 — — — — 0.0058 40 0.60 2.2 0.58 0.0100.007 0.030 0.32 0.46 0.22 — — — — 0.0039 41 0.50 2.2 0.60 0.007 0.0090.032 0.32 0.55 0.20 0.08 0.0025 — — 0.0041 42 0.52 2.2 0.50 0.009 0.0060.025 0.27 0.50 0.19 0.10 — — — 0.0039 43 0.62 2.2 0.64 0.010 0.0070.028 0.30 0.55 — 0.10 — — — 0.0055 44 0.55 2.3 0.74 0.008 0.008 0.0290.27 0.49 — 0.08 — — — 0.0054 45 0.56 2.3 0.59 0.007 0.007 0.031 0.350.56 0.34 0.07 0.0030 — — 0.0051 46 0.50 2.4 0.52 0.008 0.009 0.029 0.270.64 0.33 0.08 — — — 0.0040 47 0.59 2.2 0.49 0.009 0.008 0.024 0.28 0.600.18 0.07 — — — 0.0053 48 0.65 2.1 0.41 0.008 0.006 0.029 0.44 0.53 0.260.07 — — — 0.0043

TABLE 4 Number of Whether or not treatment for reduction Amount ofAmount inclusions of 25 of hydrogen in steel is implementednondiffusible Tensile Corrosion of O μm or more of Molten Homo- Coolingat 400 hydrogen of strength fatigue Test Steel (% by rolled materialsteel genizing to 100° C. rolled material of wire properties No. No.mass) (inclusions/100 g) treatment treatment after rolling (ppm) (MPa)(×10⁴ times) 1 1 0.0005 1 Implementation — — 0.28 1,970 27.8 2 2 0.00087 — Implementation — 0.33 1,965 20.2 3 3 0.0006 1 — — Implementation0.34 1,966 21.4 4 4 0.0009 24 Implementation ImplementationImplementation 0.05 1,950 20.2 5 5 0.0004 1 Implementation — — 0.252,152 29.8 6 6 0.0007 5 — Implementation — 0.35 2,140 21.9 7 7 0.0007 5— — Implementation 0.36 2,165 22.4 8 8 0.0008 15 ImplementationImplementation Implementation 0.04 2,155 16.8 9 9 0.0007 5Implementation — — 0.22 2,120 22.4 10 10 0.0006 1 — Implementation —0.35 2,132 22.6 11 11 0.0009 28 Implementation ImplementationImplementation 0.06 2,104 17.3 12 12 0.0008 11 Implementation — — 0.192,097 14.6 13 13 0.0008 13 Implementation — — 0.18 2,095 19.1 14 140.0007 1 Implementation — — 0.21 2,110 34.2 15 15 0.0008 18Implementation — — 0.22 2,111 20.5 16 16 0.0005 1 Implementation — —0.22 2,105 27.5

TABLE 5 Number of Whether or not treatment for reduction Amount ofAmount inclusions of 25 of hydrogen in steel is implementednondiffusible Tensile Corrosion of O μm or more of Homo- Cooling athydrogen strength fatigue Test (% by rolled material Molten genizing 400to 100° C. of rolled of wire properties No. Steel mass) (inclusions/100g) steel treatment treatment after rolling material (ppm) (MPa) (×10⁴times) 17 17 0.0005 1 Implementation — — 0.19 2,105 8.0 18 18 0.0006 2Implementation — — 0.19 2,104 8.5 19 19 0.0015 143 Implementation — —0.28 1,948 9.5 20 20 0.0021 332 Implementation — — 0.18 1,953 6.1 21 210.0012 75 Implementation — — 0.26 2,152 8.7 22 22 0.0018 220Implementation — — 0.30 2,155 6.6 23 23 0.0013 95 Implementation — —0.21 2,120 8.0 24 24 0.0025 488 Implementation — — 0.22 2,118 2.6 25 250.0006 2 — — — 0.52 2,120 6.4 26 26 0.0006 1 — — — 0.45 2,148 8.5 27 270.0004 1 — — — 0.48 2,148 9.7 28 28 0.0003 1 — — — 0.63 2,123 6.8 29 290.0008 14 — — — 0.45 2,198 8.2 30 30 0.0023 440 — — — 0.56 1.955 7.8 3131 0.0015 137 — — — 0.53 2.043 6.5

TABLE 6 Number of Whether or not treatment for reduction Amount ofAmount inclusions of 25 of hydrogen in steel is implementednondiffusible Tensile Corrosion of O μm or more of Molten Homo- Coolingat hydrogen strength fatigue Test Steel (% by rolled material steelgenizing 400 to 100° C. of rolled of wire properties No. No. mass)(inclusions/100 g) treatment treatment after rolling material (ppm)(MPa) (×10⁴ times) 32 32 0.0007 5 Implementation — — 0.23 2,032 39.4 3333 0.0004 8 Implementation — — 0.15 2,050 39.0 34 34 0.6009 13Implementation — — 0.18 2,109 46.9 35 35 0.0004 6 Implementation — —0.23 2,042 63.2 36 36 0.0007 8 Implementation — — 0.22 2,099 43.2 37 370.0005 7 Implementation — — 0.19 2,014 46.6 38 38 0.0006 6Implementation — — 0.18 2,085 48.8 39 39 0.0007 6 Implementation — —0.22 2,055 44.0 40 40 0.0004 5 Implementation — — 0.23 2,087 37.8 41 410.0007 4 Implementation — — 0.18 2,099 45.0 42 42 0.0007 4Implementation — — 0.23 2,010 40.0 43 43 0.0004 1 Implementation — —0.22 2,095 47.0 44 44 0.0006 1 Implementation — — 0.21 2,056 39.2 45 450.0007 9 Implementation — — 0.18 2,010 48.8 46 46 0.0007 5Implementation — — 0.23 2,098 51.2 47 47 0.0004 3 Implementation — —0.22 2,099 43.0 48 48 0.0004 1 Implementation — — 0.18 2,099 46.4

From these results, the following observations can be made. In thesamples of test Nos. 1 to 16 shown in Table 4 and the samples of testNos. 32 to 48 shown in Table 6, since steels in which the chemicalcomposition of a steel material is appropriately adjusted are producedunder preferable production conditions mentioned above, the number ofoxide inclusions and the amount of nondiffusible hydrogen satisfy theranges defined in the present invention. All of wires obtained bysubjecting such a wire rod to wire drawing, followed by quenching andtempering have excellent tensile strength of 1,900 MPa or more.Moreover, all of wires exhibit fatigue life of 100,000 times or moreafter quenching and tempering, and are excellent in corrosion fatigueproperties.

Whereas, the samples of test Nos. 17 to 31 shown in Table 5 are inferiorin corrosion fatigue properties because at least any one of requirementsof chemical composition of a steel material, the number of oxideinclusions and the amount of nondiffusible hydrogen defined in thepresent invention is invalid.

The samples of test Nos. 17 and 18 are examples using steels Nos. 17 ad18 that contains neither Cu nor Ni added therein, or do not meet thedefined lower limit, and thus corrosion fatigue properties weredegraded. In the samples of test Nos. 19 to 24, insufficient deoxidizingtreatment leads to excess amount of O in the steel, so that the numberof oxide inclusions in the rolled material increased and corrosionfatigue properties were degraded.

In the samples of test Nos. 25 to 29, although the amount of O in thesteel is controlled in an appropriate range, the amount of nondiffusiblehydrogen in the rolled material increased since the above-mentionednondiffusible hydrogen reducing treatment was not performed, so thatfatigue life decreased to less than 100,000 times and corrosion fatigueproperties were degraded.

In the samples of test Nos. 30 and 31, insufficient deoxidizingtreatment leads to excess amount of O in the steel, and also the numberof oxide inclusions in the rolled material since the above-mentionednondiffusible hydrogen reducing treatment was not performed. Because ofincreasing the amount of nondiffusible hydrogen in the rolled material,fatigue life decreased to less than 100,000 times and corrosion fatigueproperties were degraded.

Based on these results, an influence of the number of oxide inclusionsand the amount of nondiffusible hydrogen in the rolled material oncorrosion fatigue properties is shown in FIG. 1. In FIG. 1, inventedexamples (expressed by the symbol “o” (circle)) denote samples of testNos. 1 to 16 in Table 4 and comparative examples (expressed by thesymbol “x” (cross)) denote samples of test Nos. 19 to 31 in Table 5, andthe number of oxide inclusions in the rolled material was mentioned as“Number of Inclusions”. These results reveal that rigid definition ofthe number of oxide inclusions and amount of nondiffusible hydrogen iseffective to improve corrosion fatigue properties.

INDUSTRIAL APPLICABILITY

The rolled material and the wire of the present invention areindustrially useful since they can be suitably used for coil springsthat are used in automobiles, for example, a valve spring, a suspensionspring, and the like that are used in the engine, suspension, and thelike.

1. A rolled material for high strength spring, comprising, in % by mass:C: 0.39 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%, P: exceeding 0% and0.015% or less, S: exceeding 0% and 0.015% or less, Al: 0.001 to 0.1%,Cu: 0.10 to 0.80%, Ni: 0.10 to 0.80%, O: exceeding 0% and 0.0010% orless, and with the balance being iron and inevitable impurities,wherein: a number of oxide inclusions having an average diameter of 25μm or more is 30 or less per 100 g of a steel material; and an amount ofnondiffusible hydrogen is 0.40 ppm by mass or less.
 2. The rolledmaterial for high strength spring according to claim 1, furthercomprising, in % by mass, at least one of the following (a) to (d): (a)Cr: exceeding 0% and 1.2% or less, (b) Ti: exceeding 0% and 0.13% orless, (c) B: exceeding 0% and 0.01% or less, and (d) at least one of Nb:exceeding 0% and 0.1% or less, and Mo: exceeding 0% and 0.5% or less. 3.A wire for high strength spring, comprising in % by mass: C: 0.39 to0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%, P: exceeding 0% and 0.015% orless, S: exceeding 0% and 0.015% or less, Al: 0.001 to 0.1%, Cu: 0.10 to0.80%, Ni: 0.10 to 0.80%, O: exceeding 0% and 0.0010% or less, and ironand inevitable impurities, wherein: a number of oxide inclusions havingan average diameter of 25 μm or more is 30 or less per 100 g of a steelmaterial; and an amount of nondiffusible hydrogen is 0.40 ppm by mass orless; an area ratio of tempered martensite is 80% or more; and a tensilestrength is 1,900 MPa or more.
 4. The wire for high strength springaccording to claim 3, further comprising, in % by mass, at least one ofthe following (a) to (d): (a) Cr: exceeding 0% and 1.2% or less, (b) Ti:exceeding 0% and 0.13% or less, (c) B: exceeding 0% and 0.01% or less,and (d) at least one of Nb: exceeding 0% and 0.1% or less, and Mo:exceeding 0% and 0.5% or less.